New to me skimmer question

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I picked up a Reef Octopus Prime 200-INT this week that someone has used for the past 8 months. On top of the adjustment pimp where the brackets slide on for the silencer, the top plate is pre drilled with a circle thats smaller than the adjustment pipe. Is that correct or should it be the same size as the adjustment pipe? I am pretty sure that I saw a video where its supposed to sit on top of the pipe and slide into place. If this is wrong then i can probably drill the right size hole with a hole saw but i wanted to make sure. THanks for any help!

 

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